Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

  Plant Growth Substances/Hormones

I. Plant Hormones - do they exist?  
    You betcha. In class we will provide some case studies, including early experiments by Darwin (1880), Boysen-Jensen (1913), Paal (1918) and Went (1923) involving phototropic curvature in canary grass coleoptiles and more recent work concerning fruit set in soybean.

    Conclusion: from these studies, and countless others, we conclude that plants possess a well-developed system of chemical messengers that induce (inhibit or promote) growth and developmental responses. These chemical messengers are termed "hormones".

II. Plant hormones - What are they? They are defined as:

1. small
2. organic compounds;
3. synthesized by the plant;
4. active in low concentration (<10^-6);
5. promote or inhibit growth and developmental responses;
6. often show a separation of the site of production and the site of action (although this isn�t as clear a distinction as in animals).

Based on these criteria, would the following substances be considered hormones? Ca²⁺, sucrose, 2 4 - D, glycine, or K⁺?  

III. Plants vs. animals
    There are a few significant differences in the nature of hormones found in plants and animals. These are summarized in the following table:

    Thus, there are comparatively few plant hormones, each elicits a variety of responses and often works together with other hormones. In contrast, animals have numerous different kinds of hormones, each with a specific function, and it works alone to induce a response.

    Why the differences? Good question. It may be partly a function of our ignorance; in other words, there are likely to be many more plant hormones that simply haven�t yet been identified. Nevertheless, I suspect  that at least part of the reason for the differences is related to body design. Recall that plants have an architectural design with a limited number of parts that are repeated. It follows that only a limited number of hormones would be necessary to induce growth/developmental responses. With a mechanical design and numerous separate parts, animals required unique chemical messengers to interact with each one.

IV. Plant hormones
    There are five major groups, based on chemical structure. With the exception of the latter two, each group represents a family of related compounds. These groups are: (1) auxins; (2) gibberellins; (3) cytokinins; (4) abscisic acid; and (5) ethylene. In addition, there are a variety of other plant "hormones" including the brassinosteroids, oligosaccharides, polyamines, jasmonic acid, salicylic acid, systemin, and putative hormones like those involved in flowering (florigen). 

V. How do we know a substance is a hormone?  
    In the past, physiologists simply applied the substance to a plant or excised part to see if it caused a response. If there was a response, then we assumed that the substance was a hormone involved in the response. However, responding to an exogenous application of a hormone doesn�t necessarily mean that the substance has any endogenous (in vivo) action. Thus, we�ve become a little more picky and only accept that a substance is a hormone if:

1. Exogenous application causes the response;
2. Lowering endogenous levels prevents the response;
3. Lowering endogenous levels followed by exogenous application restores the response;
4. Endogenous levels should be related to the response (i.e., increase before the response occurs).

Check out the case study concerning bolting in Agrostemma lithago (Jones and Zeevart, 1980, Planta 149:269).

VI. Mechanism of hormone action
    Hormones act on target tissues to activate a receptor. The general mechanism is: 

hormone � target tissue/cell � receptor � signal amplification � response

Thus, for a response to occur:

1. the hormone must be present in sufficient quantity;
2. the target tissue must be sensitive (sensitized) to the hormone;
3. the target tissue recognizes the hormone (i.e., there must be a receptor to which the hormone can bind);
4. the binding of the hormone/receptor should initiate a change in the receptor (amplification).  Calcium is often involved and its interaction is mediated by the protein calmodulin; and
5. the activated receptor initiates a physiological response

VII. Techniques to study hormones

A. Bioassays
    A bioassay examines the effect of a test substance on a plant tissue. To perform a hormone bioassay, a test plant is chosen that lacks the hormone for a response. Known amounts of hormone are added to the plant, the response is measured, and a  "standard curve" is produced.  To determine if a sample contains the hormone, the test plant is treated in a similar fashion.  If present the hormone can be quantified by comparing its response to the samples of known concentration. For example, a variety of rice (Tauginbozu) lacks GA and is often used in bioassay studies. After treatment with various concentrations of GA, leaf length vs. [GA] is plotted.

Table:  Comparison of the advantages and disadvantages of bioassays
Advantages	Disadvantages
simple & easy inexpensive	sensitive to impurities false positive tests can result lower sensitivity than other methods

B. Immunological studies
    Antibodies are made against the plant hormones and then used as specific probes to localize and quantify. The antibodies are usually coupled to radioisotopes or fluorescent dyes to make it easier to trace.  This technique is very sensitivity and specific.

C. Instrumental Methods:  GC-MS; HPLC; high specificity and sensitivity

VIII. Methods for regulating endogenous levels
    The internal levels of plant hormones must be tightly controlled so that the response occurs only at the appropriate time. Regulation of hormonal action is achieved by: (1) controlling the rate of hormone synthesis; (2) forming conjugates, which are inactive storage forms where the hormone is covalently bonded to a sugar, amino acid or other molecule; (3) enzyme degradation; (4) transporting the hormone away/toward the site; and (5) compartmentalizing the substance in an organelle such as the chloroplast.

IX. Hormone Studies
    We will study each of the major groups of hormones. For each we will focus on:

1. general structure/chemistry
2. biosynthesis (pathway, site, precursors)
3. regulation of endogenous levels
4. bioassays
5. mode of transport (tissue, direction)
6. physiological actions/roles (especially important)
7. antagonists/inhibitors
8. mechanism of action,, receptors
9. commercial applications (if any)
10. cool stuff